A number of existing apparatuses are known for interfacing an external information system with the human nervous system. For example, electrocorticography (ECoG) uses multiple electrode arrays that are placed directly on the surface of a subject's brain to record electrical activity within the cerebral cortex. Because ECoG interfaces cortical tissue, ECoG is a method of interfacing with the central nervous system as a whole. An additional example of an apparatus for use in interfacing the central nervous system is a Utah array. A Utah array is a group of microscopic tines, each containing a number of electrode sites, that are inserted directly into cerebral cortical tissue. An external information system is then electrically coupled to a base portion of the array to facilitate the communication of signals from the information system to the cerebral cortical tissue and/or from the cerebral cortical tissue to the information system. Another example of an apparatus for use in interfacing the central nervous system is a spinal cord stimulation (SCS) system. The SCS system uses small leads, containing multiple electrode sites, placed in close proximity to the spinal cord to communicate, such as delivering and/or receiving, signals between the spinal cord and an external information system.
In addition, a number of apparatuses are available for use in establishing communication between an external information system and a subject's peripheral nervous system. For example, a nerve cuff electrode includes electrical contacts embedded within a conduit that is wrapped around the circumference of a nerve. Connections between the ring electrodes, electrical leads, electrical contacts, and an external information system facilitate communication between the peripheral nerve and an external device. Another example of an apparatus for use in interfacing the peripheral nervous system is a Utah slant array. The Utah slant array is similar to the Utah array discussed above, however, the microscopic tines of the slant array have varying heights that allow the tines to interact with peripheral nervous tissue at different depths of penetration.
Another example of an electrode that may be used to interface the peripheral nervous system is a sieve electrode. A sieve electrode is a thin-film device that contains numerous holes, some of which are surrounded by, or in proximity to, small metal ring electrodes. During use, the electrode is implanted between the two transected ends of a peripheral nerve or trunk, which are microsurgically attached and secured to either side of the device. Axons within the proximal end of the nerve then regenerate through the holes in the electrode, including a number of holes surrounded by, or in proximity to, metal ring electrodes, and functionally reconnect with distal motor and sensory targets. Connections between the metal ring electrodes and an external information system facilitate communication between axons within holes surrounded by metal ring electrodes and an external device. Sieve electrodes are especially useful for the communication of signals from the information system to the peripheral nervous tissue. During this process, pulses of electrical current are delivered to the peripheral nerve tissue via ring electrodes. The delivery of electrical current excites (e.g., depolarizes) local axons resulting in the initiation of action potentials, the basic unit of information within the nervous system. Unfortunately the design of current sieve electrodes is such that electrical stimulation only results in initiation of bidirectional action potentials, or action potentials that simultaneously travel along the target nerve both distally, towards the subject's target muscle or sensory organs, and proximally, towards the subject's spinal cord. Such bi-directional action potentials are particularly undesirable in neuroprosthetic applications due to their lack of directional specificity, and, therefore, functional specificity. For example, electrical stimulation of a nerve for the purpose of activating afferent sensory fibers to induce sensory percepts may result in unwanted, simultaneous motor effects. Similarly, electrical stimulation of a nerve for the purpose of activating efferent motor fibers to induce graded muscle contraction may result in unwanted, simultaneous sensory percepts or the initiation of non-specific muscle reflexes. Accordingly, a sieve electrode that provides directional specificity (i.e., can evoke unidirectional action potentials), and, therefore, functional specificity, is greatly desired.